An optical sensor combining surface plasmon resonance, light extinction, and near-critical angle reflection, for thin liquid film biochemical sensing

Abstract

We present a novel and low-cost optical sensor to obtain at the same time, fast and label-free optical information such as reflectivity, refractive index and extinction coefficient of biochemical samples. The sensor measures in around one minute optical internal reflectivity as a function of the angle of incidence at the interface between a glass prism covered by a gold nanofilm (≈20 nm) in contact with a liquid sample (≈10 µL). The nanofilm is thin enough that a large fraction of the incident light transmits through it at angles of incidence below the critical angle set by the prism and sample refractive indices (≈ 61°), but thick enough to present a Surface Plasmon Resonance dip for angles of incidence beyond that critical angle. The calculated sensitivity of SPR to the surface refractive index is 168.35 deg/RIU. We demonstrate that when a sample contains live bacteria (E. coli) the whole reflectivity curve including the SPR region is clearly affected, and to adjust a theoretical model to the experimental curves, it is necessary to assume a complex refractive index of the nanofilm appreciably different than that of gold. The optical curve of the sensor provides more information than a conventional SPR measurements offering a more complete strategy for the analysis of biochemical substances and bacterial cell suspensions.